hi, thanks for those thoughts - I think a tooth can be ruled out, though it sure looks like one, because it's light and hollow and just not tooth material. An air bladder - had to look this up on Wiki, but I'd imagine it would have soft flexible walls? this thing hasn't collapsed out of the water, so it has some structure to it. My original thought was also an egg, but of what?? weird shape, but then so are some critters. Any further thoughts/info welcome